Showing papers on "Thermal contact conductance published in 2010"
TL;DR: In this article, effective thermal conductivity models of nanofluids are reviewed and comparisons between experimental findings and theoretical predictions are made, and the results show that there exist significant discrepancies among the experimental data available and between the experimental results and the theoretical model predictions.
Abstract: Adding small particles into a fluid in cooling and heating processes is one of the methods to increase the rate of heat transfer by convection between the fluid and the surface. In the past decade, a new class of fluids called nanofluids, in which particles of size 1–100 nm with high thermal conductivity are suspended in a conventional heat transfer base fluid, have been developed. It has been shown that nanofluids containing a small amount of metallic or nonmetallic particles, such as Al2O3, CuO, Cu, SiO2, TiO2, have increased thermal conductivity compared with the thermal conductivity of the base fluid. In this work, effective thermal conductivity models of nanofluids are reviewed and comparisons between experimental findings and theoretical predictions are made. The results show that there exist significant discrepancies among the experimental data available and between the experimental findings and the theoretical model predictions.
550 citations
TL;DR: In this paper, an equilibrium molecular-dynamics (EMD) simulation is performed to systematically study the strain effects on the lattice thermal conductivity of low-dimensional silicon and carbon materials: silicon nanowires (one dimensional), thin-films (two dimensional), single-walled carbon nanotube (SWCNT, one dimensional) and single-layer graphene sheet (twodimensional).
Abstract: Applying stress/strain on a material provides a mechanism to tune the thermal conductivity of materials dynamically or on demand. Experimental and simulation results have shown that thermal conductivity of bulk materials can change significantly under external pressure (compressive stress). However, stress/strain effects on the thermal conductivity of nanostructures have not been systematically studied. In this paper, equilibrium molecular-dynamics (EMD) simulation is performed to systematically study the strain effects on the lattice thermal conductivity of low-dimensional silicon and carbon materials: silicon nanowires (one dimensional) and thin-films (two dimensional), single-walled carbon nanotube (SWCNT, one dimensional) and single-layer graphene sheet (two dimensional). Spectral analysis of EMD is further developed and then applied to avoid the numerical artifacts such as the neglect of long-wavelength phonons that are often encountered when using EMD with periodic boundary conditions. Intrinsic thermal conductivity of the simulated bulk and nanostructures can be obtained using spectral analysis of EMD. The thermal conductivity of the strained silicon and diamond nanowires and thin films is shown to decrease continuously when the strain changes from compressive to tensile. However, for SWCNT and single-layer graphene, the thermal conductivity has a peak value, and the corresponding applied strain is at $\ensuremath{-}0.06$ or $\ensuremath{-}0.03$ for SWCNTs depending on the chirality and at zero for graphene, respectively. The following two reasons could explain well the effects of strains on the thermal conductivity of the nanowires and thin films that decreases continuously from compressive strain to tensile strain: (1) mode-specific group velocities of phonons decrease continuously from compressive strain to tensile strain and (2) the specific heat of each propagating phonon modes decrease continuously from compressive strain to tensile strain. However, for SWCNT and single-layer graphene, the mechanical instability induces buckling phenomenon when they are under compressive strains. The phonon-phonon scattering rate increases significantly when the structure buckles. This results in the decreasing behavior of thermal conductivity of SWCNT and graphene under compressive stress and explains the peak thermal conductivity value observed in SWCNT and single-layer graphene when they are under strain. The results obtained in this paper has important implications of challenging thermal management of electronics using advanced materials such as carbon nanotubes and graphene. It also points to a potentially new direction of dynamic thermal management.
401 citations
TL;DR: In this article, a review of the literature describing the packing structure and effective thermal conductivity of randomly packed beds consisting of mono-sized particles is presented, with particular attention paid to the packing structures (porosity, coordination number, and contact angles) and heat transfer.
332 citations
TL;DR: In this paper, the authors measured the thermal properties of VO2 thin films, 90-440 nm thick, across the metal-insulator transition (MIT), and found that the thermal conductivity increases by as much as 60% in the metallic phase.
Abstract: The thermal properties of VO2 thin films, 90–440 nm thick, are measured by time-domain thermoreflectance (TDTR) across the metal-insulator transition (MIT). The thermal conductivity increases by as much as 60% in the metallic phase; this increase in conductivity is in good agreement with the expected electronic contribution to the thermal conductivity. For relatively thick layers, TDTR data are sensitive to the dynamic heat capacity and show a pronounced peak near the MIT temperature created by a contribution to the enthalpy from periodic transformations at the 10 MHz frequency of the thermal waves used in the experiment. The dynamic heat capacity increases as the amplitude ΔT of the thermal waves becomes comparable to the width of the MIT and reaches ≈30% of the bulk latent heat for ΔT≈1.6 K.
188 citations
TL;DR: In this article, the authors measured the room-temperature thermal boundary conductance of aluminum films grown on silicon substrates subjected to various pre-Al-deposition surface treatments with a pump-probe thermoreflectance technique.
Abstract: In nanosystems, the primary scattering mechanisms occur at the interfaces between the material layers. As such, the structure and composition around these interfaces can affect scattering rates and, therefore, thermal resistances. In this work, we measure the room-temperature thermal boundary conductance of aluminum films grown on silicon substrates subjected to various pre-Al-deposition surface treatments with a pump-probe thermoreflectance technique. The Si surfaces are characterized with atomic force microscopy to determine mean surface roughness. The measured thermal boundary conductances decrease as Si surface roughness increases. In addition, stripping of the native oxide layer from the surface of the Si substrate immediately prior to Al film deposition causes the thermal boundary conductance to increase. The measured data are compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface in order to better elucidate the thermal scattering processes affecting thermal boundary conductance at rough interfaces.
161 citations
TL;DR: In this article, the experimental measurements of contact thermal resistance between individual carbon nanotubes were performed and it was shown that the contact thermal conductance can increase by nearly two orders of magnitude (from 10−8 to 10−6 W/K) as the contact area increases from a cross contact to an aligned contact.
Abstract: We report on experimental measurements of contact thermal resistance between individual carbon nanotubes. Results indicate that the contact thermal conductance can increase by nearly two orders of magnitude (from 10−8 to 10−6 W/K) as the contact area increases from a cross contact to an aligned contact. Normalization with respect to the contact area leads to normalized contact thermal resistance on the order of 10−9 m2 K/W at room temperature, one order of magnitude lower than that from a molecular dynamics simulation in literature. These results should have important implications in the design of carbon nanotube-polymer composites for tunable thermal properties.
158 citations
TL;DR: In this paper, the all-atom model molecular-dynamics simulation is conducted to investigate the tuning of polymer thermal conductivity using mechanical strains, and the simulation results show that the thermal conductivities of polymers increases with the increasing strain and the enhancement is larger when the polymer is stretched slower.
Abstract: The low thermal conductivity of polymers limits their heat spreading capability, which is one of the major technical barriers for the polymer-based products, especially electronics, such as organic light emitting diodes. It is highly desirable to enhance the thermal conductivity of polymer materials including polymer composites. Mechanical stretching could align polymer chains which are intrinsically low-dimensional material that could have very high thermal conductivity and thus enhancing the thermal conductivity of polymers. In this work, the all-atom model molecular-dynamics simulation is conducted to investigate the tuning of polymer thermal conductivity using mechanical strains. The simulation results show that the thermal conductivity of polymers increases with the increasing strain and the enhancement is larger when the polymer is stretched slower. Molecular weight also affects the thermal conductivity under the same stretching condition. More importantly, the thermal-conductivity enhancement could be exponentially fitted with the orientational order parameter which describes the chain conformation change. This study could guide the development of advanced reconfigurable and tunable thermal management technologies.
153 citations
TL;DR: In this paper, the thermal properties of polytetrafluoroethylene (PTFE) were analyzed using various thermal analysis and thermophysical properties test techniques, such as differential scanning calorimetry and pushrod dilatometry.
Abstract: Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer used in numerous industrial applications. It is often referred to by its trademark name, Teflon. Thermal characterization of a PTFE material was carried out using various thermal analysis and thermophysical properties test techniques. The transformation energetics and specific heat were measured employing differential scanning calorimetry. The thermal expansion and the density changes were determined employing pushrod dilatometry. The viscoelastic properties (storage and loss modulus) were analyzed using dynamic mechanical analysis. The thermal diffusivity was measured using the laser flash technique. Combining thermal diffusivity data with specific heat and density allows calculation of the thermal conductivity of the polymer. Measurements were carried out from − 125 °C up to 150 °C. Additionally, measurements of the mechanical properties were carried out down to − 170 °C. The specific heat tests were conducted into the fully molten regions up to 370 °C.
141 citations
TL;DR: In this article, the effects of tortuosity and dispersion on the effective thermal conductivity of fluid-saturated porous media are investigated analytically with help of a volume averaging theory.
113 citations
TL;DR: In this paper, the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the GDL materials and an electrolytic iron surface as a function of compression load and PTFE content at 70°C were investigated.
111 citations
TL;DR: In this paper, the effective thermal conductivity of carbon paper diffusion media was estimated numerically using GeoDict code and the governing mathematical equations were solved using the commercial software package Fluent (6.3.26) and the ThermoDict solver.
TL;DR: In this article, the authors proposed a 3.5 mm-thick Ti-based flat heat pipe for thermal ground plane (TGP) applications, which is constructed by laser welding two microfabricated titanium substrates to form a hermetically sealed vapor chamber.
Abstract: A novel 3 cm × 3 cm × 600 μm-thick Ti-based flat heat pipe is developed for Thermal Ground Plane (TGP) applications. The Ti-based heat pipe architecture is constructed by laser welding two microfabricated titanium substrates to form a hermetically sealed vapor chamber. The scalable heat pipes' flat geometry facilitates contact with planar heat sources, such as microprocessor chip surfaces, thereby reducing thermal contact resistance and improving system packaging. Fluid transport is driven by the wicking structure in the TGP, which consists of an array of Ti pillars that are microfabricated from a titanium substrate using recently developed high-aspect-ratio Ti processing techniques. The hydrophilic nature of the Ti pillars is increased further by growing ~200-nm hairlike nanostructured titania of the pillar surfaces. The resulting super hydrophilic wick offers the potential to generate high wicking velocities of ~27.5 mm/s over distances of 2 mm. The experimental wetting results show a diffusive spreading behavior that is predicted by Washburn dynamics. The maximum effective thermal conductivity of a heat pipe is directly related to the speed of capillary flow of the working fluid through the wick and is measured experimentally in the first-generation device to be k = 350 W/m · K. A dummy TGP with a cavity volume of ~170 μL was used to test the hermiticity level of the laser packaging technique. The device gave a 0.067% of water loss based on ~60 μL of charged water at 100°C in air for over a year.
TL;DR: In this article, the thermal conductivity of polyethylene with molecular dynamics simulations is investigated. And the results are important for designing inexpensive high thermal-conductivity polymers, which gives rise to an interesting one-dimensional-to-three-dimensional transition in phonon transport.
Abstract: The thermal conductivity of nanostructures generally decreases with decreasing size because of classical size effects. The axial thermal conductivity of polymer chain lattices, however, can exhibit the opposite trend, because of reduced chain-chain anharmonic scattering. This unique feature gives rise to an interesting one-dimensional-to-three-dimensional transition in phonon transport. We study this transition by calculating the thermal conductivity of polyethylene with molecular dynamics simulations. The results are important for designing inexpensive high thermal-conductivity polymers.
TL;DR: In this article, a 3D model of the heat transfer in the system has been established and simulated to determine the validity conditions of a 1D model to represent the center temperature.
Abstract: This paper presents a method dedicated to thermal conductivity measurement of thin (a few millimeters thickness) insulating and super-insulating materials. The method is based on the measurement of the temperature at the center of a heating element inserted between two samples, with the unheated surface of the samples maintained constant. A 3D model of the heat transfer in the system has been established and simulated to determine the validity conditions of a 1D model to represent the center temperature. This 1D model was then used to realize a sensitivity analysis of the center temperature to the different parameters. The conclusion is that the thermal conductivity may be estimated with a good precision for all insulating materials from a simple steady state measurement and that the thermal capacity may also be estimated from transient recording of the temperature with a precision increasing with the value of the thermal capacity of the samples. It has then been shown that a device with two samples of different thickness improves the precision of the estimation of the thermal capacity. These conclusions are validated by an experimental study on polyethylene foam and PVC samples leading to an estimation of their thermal properties very close to the values measured by other classical methods (deviation < 5%).
TL;DR: In this paper, the heat capacity, thermal conductivity, thermal diffusivity, and thermal expansion of Bi4Ge3O12 single crystals have been measured over a wide temperature range.
Abstract: The heat capacity, thermal conductivity, thermal diffusivity, and thermal expansion of Bi4Ge3O12 single crystals have been measured over a wide temperature range.
TL;DR: In this article, thermal conductance values at interfaces between aluminum and diamond with various surface terminations over a range of temperatures from 88 to 300 K were reported, and it was shown that conductance at oxygenated diamond interfaces is roughly four times higher than at hydrogen-treated diamond interfaces.
Abstract: Synthetic diamond has potential as a heat spreading material in small-scale devices. Here, we report thermal conductance values at interfaces between aluminum and diamond with various surface terminations over a range of temperatures from 88 to 300 K. We find that conductance at oxygenated diamond interfaces is roughly four times higher than at hydrogen-treated diamond interfaces. Furthermore, we find that Al grain structure formation is not strongly dependent on diamond surface chemistry, which suggests that interfacial bonding influences thermal conductance. The results reported here will be useful for device design and for advancing models of interfacial heat flow.
01 Jan 2010
TL;DR: In this paper, the throughplane thermal conductivities of several widely used carbon porous transport layers (PTLs or GDLs) and their thermal contact resistance to an aluminium polarisation plate were reported.
Abstract: We report the through-plane thermal conductivities of the several widely used carbon porous transport layers (PTLs or GDLs) and their thermal contact resistance to an aluminium polarisation plate. We report these values both for wet and dry samples and at different compaction pressures. We show that depending on the type of PTL and possible residual water, the thermal conductivity of the materials varies from 0.15 to 1.6 W K−1 m−1 — one order of magnitude. This behaviour is the same for the contact resistance varying from 0.8 to 11 10−4 m2 K W−1 . For dry PTLs the thermal conductivity decreases with increasing PTFE content and increases with residual water. These effects are explained by the behaviour of air, water and PTFE in between the PTL fibres.Copyright © 2010 by ASME
TL;DR: In this article, a three-dimensional random network model is proposed to evaluate the thermal conductivity of particulate materials, which is applied to numerical assemblies of poly-dispersed spheres generated using the discrete element method (DEM).
TL;DR: In this article, a photothermal technique is used to measure the thermal effusivity, thermal diffusivity and specific heat capacity of a few representative species of plant leaves and compared with the corresponding properties of two materials abundant in the environment.
Abstract: It is known that the thermal properties of a material influence the temperature around it. Once heated, the rate at which a material transfers the absorbed heat into the surroundings is determined by the thermal effusivity (or thermal inertia) of the material, and it depends on the well-known thermal properties, thermal conductivity, and specific heat capacity. Since a direct measurement of these properties is rather difficult for thin biological specimens such as plant leaves, a photothermal technique is used to measure the thermal effusivity, thermal diffusivity, thermal conductivity, and specific heat capacity for a few representative species of plant leaves. Measurements have been carried out on fresh as well as dry leaves to estimate the differences in their properties. Thermal properties of plant leaves are compared with the corresponding properties of two materials abundant in the environment and discussed. The influence of thermal properties, particularly the thermal effusivity and specific heat capacity, of plant leaves on controlling the temperature of the environment around them is discussed.
TL;DR: In this paper, the effects of surface modification and particle size on the performance of thermally conductive epoxy adhesives were investigated, and it was revealed that epoxy composites filled with nanosized particles performed higher thermal conductivity, electrical insulation, and mechanical strength than those filled with microsized ones.
Abstract: Microsized or nanosized α-alumina (Al2O3) and boron nitride (BN) were effectively treated by silanes or diisocyanate, and then filled into the epoxy to prepare thermally conductive adhesives. The effects of surface modification and particle size on the performance of thermally conductive epoxy adhesives were investigated. It was revealed that epoxy adhesives filled with nanosized particles performed higher thermal conductivity, electrical insulation, and mechanical strength than those filled with microsized ones. It was also indicated that surface modification of the particles was beneficial for improving thermal conductivity of the epoxy composites, which was due to the decrease of thermal contact resistance of the filler-matrix through the improvement of the interface between filler and matrix by surface treatment. A synergic effect was found when epoxy adhesives were filled with combination of Al2O3 nanoparticles and microsized BN platelets, that is, the thermal conductivity was higher than that of any sole particles filled epoxy composites at a constant loading content. The heat conductive mechanism was proposed that conductive networks easily formed among nano-Al2O3 particles and micro-BN platelets and the thermal resistance decreased due to the contact between the nano-Al2O3 and BN, which resulted in improving the thermal conductivity. POLYM. ENG. SCI., 50:1809–1819, 2010. © 2010 Society of Plastics Engineers
TL;DR: In this article, the TCR is the dominant component of the gas diffusion layer (GDL) total thermal resistance with a significant hysteresis resulting in up to a 34% difference between the loading and unloading cycle data.
TL;DR: In this article, an analytical approach of transient heat conduction in two-layered material, of finite depth, with an imperfect thermal contact, subjected to a moving gaussian laser beam was developed.
TL;DR: In this article, the authors present new governing equations for non-Fourier heat conduction in nanomaterials based on the concept of thermomass and predict the effective thermal conductivities of nanotubes.
TL;DR: In this article, a thermal interface material (TIM) was fabricated by synthesizing aligned carbon nanotubes (CNTs) on both sides of a thin copper foil, and the Hot Disk ® method was applied to measure the thermal conductivity of these CNT-TIMs.
TL;DR: In this article, the authors measured thermal diffusivity and thermal contact resistance at the material interface for carbon/carbon composites (C/C) joined to copper by using the laser flash method.
TL;DR: In this article, the variations of thermal conductivity with temperature for Al-[x] wt.% Cu, x = 3, 6, 15 and 24 alloys were measured by using a radial heat flow apparatus.
TL;DR: In this article, the authors compared the thermal conductivity of a composite made using the equivalent inclusion method (EIM) and the finite element method (FEM) using representative volume elements.
TL;DR: In this paper, the effect of the properties of the two phases on the effective thermal conductivity of the heterogeneous system was examined and a model was presented for calculating the thermal conductivities of nanoparticles.
Abstract: This work reviews experimental data and models for the thermal conductivity of nanoparticle suspensions and examines the effect of the properties of the two phases on the effective thermal conductivity of the heterogeneous system. A model is presented for the effective thermal conductivity of nanofluids that takes into account the temperature dependence of the thermal conductivities of the individual phases, as well as the size dependence of the thermal conductivity of the dispersed phase. We demonstrate that this model can be used to calculate the thermal conductivity of nanofluids over a wide range of particle sizes, particle volume fractions, and temperatures. The model can also be used to validate experimental thermal conductivity data for nanofluids containing semiconductor or insulator particles and confirm the size dependence of the thermal conductivity of nanoparticles. © 2010 American Institute of Chemical Engineers AIChE J, 2010
TL;DR: The surface of ceramics was biomimetically roughened into nanofinned surface that creates a thin air layer enveloping the surface of the ceramic during quenching so that the strong thermal gradient and stresses produced by the steep temperature difference in thermal shock did not occur both on the actual surface and in the interior of the Ceramics.
Abstract: We propose here a new method to make ceramics insensitive to thermal shock up to their melting temperature. In this method the surface of ceramics was biomimetically roughened into nanofinned surface that creates a thin air layer enveloping the surface of the ceramics during quenching. This air layer increases the heat transfer resistance of the surface of the ceramics by about 10 000 times so that the strong thermal gradient and stresses produced by the steep temperature difference in thermal shock did not occur both on the actual surface and in the interior of the ceramics. This method effectively extends the applications of existing ceramics in the extreme thermal environments.
TL;DR: In this paper, equilibrium molecular dynamics simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions, which consisted of alkanedithiol (S−(CH 2 ) n ―S―) molecules.
Abstract: In this paper, equilibrium molecular dynamics simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions. The SAM consisted of alkanedithiol (―S―(CH 2 ) n ―S―) molecules. The out-of-plane (z-direction) thermal conductance and in-plane (x- and y-direction) thermal conductivities were calculated. The simulation finite size effect, gold substrate thickness effect, temperature effect, normal pressure effect, molecule chain length effect, and molecule coverage effect on thermal conductivity/ conductance were studied. Vibration power spectra of gold atoms in the substrate and sulfur atoms in the SAM were calculated, and vibration coupling of these two parts was analyzed. The calculated thermal conductance values of Au-SAM-Au junctions are in the range of experimental data on metal-nonmetal junctions. The temperature dependence of thermal conductance has a similar trend to experimental observations. It is concluded that the Au-SAM interface resistance dominates thermal energy transport across the junction, while the substrate is the dominant media in which in-plane thermal energy transport happens.